1. Field of the Invention
The present invention relates to electrophotography, and in more detail to an improved liquid developer for converting electrostatic latent images to visible images in an electrophotographic process or in an electrostatic recording process. This improved developer is especially preferred for developing electrostatic latent images on an insulating surface by the use of toner particles which are positively charged.
2. Description of the Prior Art
In an electrophotographic process, a surface of the recording element used comprising a sensitive layer which comprises photoconductive zinc oxide on a support having a comparatively high conductivity is uniformly negatively charged in the dark. An optical image of an original is then projected onto the charged surface of the sensitive layer, whereby the uniformly charged surface is partially discharged, corresponding to the intensity of the light applied, to form an electrostatic latent image. By applying electroscopic toner particles to this electrostatic latent image, a visible image is produced. In the electrofax process, this visible image is fixed directly to the photoconductive surface.
It is necessary, in common reproduction systems, to obtain a positive copy from a positive original.
Accordingly, when the surface of the sensitive layer is negatively charged, the electroscopic toner particles are positively charged, and it is preferred that the positive charge be strong and stable.
Many kinds of liquid developers containing toner particles having a positive charge have been marketed. However, these marketed developers are intended to be used for copying line or dotted images, and thus are not suitable for reproducing continuous gradation images. When continuous gradation images are reproduced using the hitherto marketed liquid developers or liquid developers having formulations known in the art, faults such as the following are observed:
1. Sufficient image density is not obtained;
2. Flowing of toner (so-called streaking) is caused on the resulting image, and
3. Fog is formed due to deposition of the toner on non-image parts.
In order to reproduce a continuous gradation image, it is generally desirable that the optical density of the image be above 2.0 and the optical density of fog be below 0.15 and that "streaking" of the image not occur. With a liquid developer, it is required that the dispersion of the toner particles be stable for a long period of time and that the positive charge on the toner neither decrease nor change into the opposite polarity.
British patent specification No. 1,093,081 discloses copolymers of a half-alkylamide and diisobutylene. No usage in an electrophotographic environment is suggested.
3. Objects of the Invention
An object of the present invention is to improve the above-mentioned faults of prior art liquid developers and to provide an excellent liquid developer by which a good continuous gradation image is obtained.
Other objects will be apparent to those skilled in the art as the description of this invention proceeds.